1. Field of the Invention
The present invention relates generally to improving the performance of magnetic storage media reading devices. More specifically, the invention relates to gain compensation for thermal asperity correction. In particular, the invention relates to a method of adding a gain compensation factor to a variable gain amplifier that compensates for a lowered input resistance at the input of the variable gain amplifier where the decreased input resistance to the variable gain amplifier was introduced for the purpose of correcting for a thermal asperity event.
2. Relationship to the Art
In the modem disk drives that employ magnetoresistive (MR) recording heads, one common problem that leads to errors or unreadable data is so called "thermal asperity" (thermal asperity) effects. A magnetoresistive head is a device which has a variable resistance in the presence of a variable magnetic field. Thermal asperity effects are caused by media defects or particulate impacting the MR head. Such impacts generate heat which can cause the MR head resistance to change. Changes in the MR head resistance caused by thermal events caused by impacts can either wash out a read signal or can appear to be an intended variation in a read signal. Thus, thermal asperity effects can impair the signal read from the disk and can cause errors during read back.
Because the effect of a thermal asperity event is similar to a low frequency baseline DC shift superimposed on the original signal, one method of compensating for such events is to attempt to remove the DC shift when a thermal asperity event is detected. This may be accomplished by lowering the input resistance of an amplifier that amplifies the signal from the MR head as is described below.
FIG. 1 is a block diagram illustrating a typical read channel for a magnetic disk reader that uses a magnetoresistive head. A disk 100 is read using a magnetoresistive head 102. By passing a constant current through the variable resistance and measuring the voltage across the variable resistance, it is possible to determine the state of the magnetic field in the region of the disk being read. The voltage output from the magnetoresistive head 102 is input into a preamplifier 104. Preamplifier 104 outputs a differential signal that is coupled to a read channel amplifier 106 using a pair of coupling capacitors 108. The interface between the preamplifier and the read channel amplifier is shown in greater detail in FIG. 2.
FIG. 2 is a block diagram illustrating the circuit which couples a preamplifier 210 to a read channel amplifier circuit 220. Typically, read channel amplifier circuit 220 includes a variable gain amplifier. The variable gain amplifier adjusts the incoming signal magnitude to realize the best signal quality. Typically, that is done in the digital domain. Vs represents the preamplifier signal that is obtained by reading the MR head, "Rs" is the output resistance of the preamplifier, which is represented by a pair of internal output resistors 212. A pair of coupling capacitors 214 couple the signal from the preamplifier to the read channel amplifier circuit 220 on a line 222 and a line 224. The coupling capacitance is chosen to be large enough so that the lowest frequency of interest may be passed without much attenuation.
The input resistance of the read channel amplifier "Rin" is represented by internal input resistor 240. The signal that is input to the read channel amplifier is roughly proportional to Rin/(Rs+Rin). Typically the Rin is much larger than Rs, so that most of the signal from the preamplifier is delivered to the read channel amplifier with very little loss.
Because the read channel amplifier circuit 220 is coupled to the preamplifier chip through capacitors 214, the DC difference between the two chips is stored on the coupling capacitors. When a thermal asperity event happens, the coupling capacitors are charged up due to the DC baseline shift. To restore the DC quickly, differential input resistance 240 on the read channel amplifier side may be lowered to reduce the input time constant and quickly discharge the coupling capacitors to the correct DC levels. One problem associated with the lowering the input resistance in this manner during a thermal asperity event is that lowering the input resistance to the read channel amplifier causes the signal to be attenuated at the same time, which causes the overall gain of the read channel to change. Thus, even though the DC can be quickly restored, the signal is distorted due to lowered input resistance.
The input resistance of variable gain amplifier 260 may be lowered by simply adding a shunt resistor 242 between line 222 and 224. As noted above, this reduces the charging and discharging time of Cin and also reduces the input resistance of variable gain amplifier 260, which reduces the signal input to the read channel amplifier. It should be noted that this effect generally is not compensated for digitally by the variable gain amplifier because the VGA gain is actually frozen during a thermal asperity event. The VGA is frozen because the incoming signal is being disturbed too much at the time for gain loop updates to be performed.
What is needed is a way of compensating for the decreased signal gain in the read channel that is caused by lowering the input resistance of the read channel amplifier.